The present invention relates generally to the field of aluminide coatings diffused onto metal substrates and particularly to targeting the diffusion of the coating to a selected area of the substrate.
Diffusing aluminide coatings onto the surface of metal gas turbine components, such as blades, vanes, combustor cases and the like, is a standard way of reducing the untoward effects of oxidation and corrosion on these components, thereby maintaining their useful life. Specifically, aluminide coatings extend the service life of a part used for operation at temperatures usually above 649xc2x0 C. (1200xc2x0 F.). Such parts are usually made from nickel or from nickel or cobalt based alloys.
Essentially, all aluminum diffusion coating methods share some common steps for accomplishing the coating: first, the coating material is placed near or in contact with the metal substrate; the coating material and substrate are then heated until the coating material diffuses onto the substrate. More specifically, the placement step involves placing the metal substrate in a retort chamber with a source of aluminum and a halide activator. The source of aluminum may be pure aluminum or an aluminum-rich intermetallic compound such as a chromium-aluminum alloy or CO2Al5 and the like. The activator may be any number of halide compounds, including an aluminum halide, alkali metal halide, ammonium halide, or mixture thereof. The activator functions to facilitate the deposition of aluminum onto the surface of the metal component.
High heat is then applied to the metal substrate, aluminum source and activator in the retort chamber for a period that ranges from two to twelve hours in an inert atmosphere to prevent the occurrence of oxidation. During the heating step, the halide activator dissociates and reacts with aluminum metal ions from the aluminum source to form Al-halide intermediates, which migrate to the surface of the metal substrate. The Al-halide intermediates xe2x80x9cgrabxe2x80x9d the metal atoms of the metal substrate. These atoms reduce the Al-halide intermediates to create intermetallic compounds, such as Ni2Al3, NiAl or NiAl3, on and at some depth below the surface of the metal substrate. These intermetallic compounds are aluminides and are generally resistant to high temperature degradation. They are consequently preferred as protective coatings.
Diffusion aluminide coating methods also share a second commonality, called activity or throwing power, which stems from the use of a halide activator. Throwing power relates to the strength of the halide activator in reacting with the aluminum ions in the aluminum source. Throwing power is essentially a measure of the potential that a halide activator has in facilitating a coating reaction. Those halide activators with greater throwing power form more reactive Al-halide intermediates. Accordingly, they can more readily pull the metal atoms of the substrate out of their crystalline structure as well as pull out metal atoms from deeper in the substrate. Halide activators with greater throwing power are able to facilitate a stronger coating reaction, which in turn relates to the thickness of the deposited coating.
Diffusion aluminide coatings thus depend on the chemical reactivity between the aluminum-halide intermediate and the metal atoms of the substrate, which, as just discussed, is a function of the reactivity of the halide activator. Other factors that affect the depth and quality of the coating include the heating temperature and the presence of any other material placed either in the heating chamber or on the surface of the substrate that could inhibit the throwing power of the halide activator.
Essentially, the differences between the various diffusion coating methods relate to the distance in placement and to the proximal relationship between the coating material and the substrate. Historically, aluminide coatings have been formed by the so-called xe2x80x9cpack cementationxe2x80x9d method described in U.S. Pat. No. 3,257,230 to Wachtell et al., and U.S. Pat. No. 3,544,348 to Boone. In this method, the metal substrate is buried in a coating material in powder form that contains an aluminum source and halide activator. That is, the coating material has an in-contact relation with the substrate. Other in-contact coating media include coating tape and slurry. Because the media is applied directly to the surface to be treated, these methods represent variants of the pack cementation method. In fact, U.S. Pat. No. 5,334,417 to Rafferty et al. discusses using coating tape to form a pack cementation-style coating on a metal surface. U.S. Pat. No. 6,045,863 to Olson et al. employs a coating tape that produces a two-zone diffusion coating. U.S. Pat. No. 5,674,610 to Schaeffer et al. uses a coating tape to perform a chromium, not aluminide, diffusion coating. U.S. Pat. No. 4,004,047 to Grisik features a coating tape in which the aluminum source is a Fexe2x80x94Al powder mixture. Also, U.S. Pat. No. 6,110,262 to Kircher et al. discloses a slurry for diffusion aluminide coating.
Somewhat different from the pack cementation method is the so-called xe2x80x9cabove-the-packxe2x80x9d coating method in which the metal substrate lies in a retort chamber apparatus above the coating material. The coating material is typically in powder form, and has an out-of-contact relation with the substrate. Besides an aluminum source and halide activator, the coating material may contain an oxide and modifier as required to reduce the activity of the halide activator. See e.g., U.S. Pat. No. 4,132,816 to Benden et al.; U.S. Pat. No. 4,148,275 to Benden et al.; U.S. Pat. No. 4,501,766 to Shankar et al., and U.S. Pat. No. 5,217,757 to Milianik et al. Essentially, these references describe vapor aluminide diffusion, whereby internal features of a metal part may be coated. A further variation is the chemical vapor deposition method of U.S. Pat. No. 5,658,614 described in Basta et al.
A problem in the use of diffusion aluminide coating for gas turbine engine parts has been the inability to consistently attain uniform coatings of inaccessible or hard to reach sections of the part to-be-coated. Methods that require in-contact relation between coating medium and the metal substrate cannot coat an inaccessible section, regardless of whether the medium is in powder form, a tape or a slurry.
The amount of coating medium applied to the substrate surface usually affects the diffused coating thickness. Previous in-contact coating methods result in a hit or miss approach to the application of coating medium for hard to for reach sections of the part. However, depending on the geometry and the irregularity of the section to be coated, using an in-contact coating mechanism such as a powder or slurry for hard to reach sections of the part likely results in an uneven coating layer applied to the substrate. In many instances the best that can be done to deliver coating medium to the hard to reach metal substrate is to estimate that an in-relation contact has been made. Further, disposing a slurry on a hard to reach part risks undetected or uncontrollable contact onto sections of the part that ought not be coated. Detecting a spotty or uneven application of the coating medium may be difficult. Moreover, when an undetectably uneven application of coating medium has been heated, detecting a non-uniform coating thickness is difficult.
Aluminide diffusion methods that allow an out-of-contact relation, such as above-the-pack cementation or vapor diffusion, may provide somewhat more control than in-contact methods. This is because in the above methods, diffusion coating occurs as a result of the entire surface of the part being automatically exposed to the aluminum vapor in the heating chamber. For example, relative to hard to reach surfaces, above-the-pack cementation has provided a way to deposit a metallic coating on internal surfaces of hollow articles, such as gas turbine blades and vanes. See U.S. Pat. No. 4,148,275 to Benden et al. Hollow gas turbine blades are placed in a chamber atop that in which the coating medium is placed. The coating medium, a powder, is heated to a temperature at which the Al halides vaporize and are directed into the blade hollows. See also U.S. Pat. No. 4,132,816 to Benden et al.
The Benden method, however, is quite limited and is useful only when coating the entire internal surface of hollow turbine engine parts is desired. This method requires specialized apparatus adapted so that the coating vapor may be pumped into the blade hollows. Such a specialized method is not readily applicable for localized repair of the aluminide coating of sections of turbine engine parts. Nor is the specialized Benden method and apparatus readily applicable for coating specific types of external features of a turbine engine part such as edge seals and platform underside pockets, which present no hollows into which vapor coating may be pumped, but bisect the blade.
Other attempts at localized aluminide diffusion coating rely only on an in-contact relation between the coating medium and substrate. See e.g., U.S. Pat. No. 6,045,863 to Olson et al. for applying a coating tape directly to the substrate surface to be repaired; U.S. Pat. No. 5,334,417 to Rafferty et al. for applying a coating tape to a localized area of metal substrate to be repaired; U.S. Pat. No. 5,658,614 to Basta et al. for applying a localized coating of platinum as a pretreatment to a section of a turbine blade to be repaired, then subjecting the blade to vapor diffusion to create a uniform coating over the pre-treated area. See also, U.S. Pat. No. 6,203,847 to Conner et al. U.S. Pat. No. 6,274,193 to Rigney et al. None of the cited references describes a method for aluminide coating of uneven or irregular surfaces.
Currently needed is a method for producing a targeted diffusion aluminide coating suitable for both first time and repair coating of hard to reach surfaces, particularly surfaces of turbine engine parts. Such a method should be capable of producing first time or repair coating on irregular surfaces. In addition, such a targeted aluminide coating method should minimize the possibility that non-targeted laterally adjacent areas of the substrate will also be coated during the localized process. The localized coating method needed will rely on an out-of-contact relation between the coating medium and substrate.
The present invention provides methods and compositions for forming an aluminide coating on a target surface of a metal substrate which is otherwise not easily accessible. The target surface bounds a contained spaced of the substrate. The present method is particularly useful when only a small portion of a metal substrate requires coating, and when extensive masking of the substrate would otherwise be required to apply coatings using conventional processes.
According to one embodiment of the invention, a method for forming an aluminide coating on a target surface of a metal substrate is provided. The target surface bounds a contained space of the substrate. The method comprises positioning a coating tape over the contained space to at least partially enclose said contained space. The coating tape is in out-of-contact relation with the target surface. The coating tape comprises a mixture comprising: (i) at least one aluminum source comprising from about 70% to about 99% by weight of the mixture, the aluminum source containing from about 20 wt. % to about 60 wt. % aluminum; and (ii) at least one halide activator comprising from about 1% to about 15% by weight of the mixture. The coating tape further comprises at least one binder. The target surface is heated to a temperature effective to cause the aluminum source to react with the activator and the target surface, thereby forming an aluminide coating on the target surface.
According to another embodiment of the invention, a method for forming an aluminide coating on a target surface of a metal substrate is provided. The target surface bounds a contained space of the substrate. The method comprises positioning a tape over the contained space to at least partially enclose said contained space, but in out-of-contact relation with the target surface. A slurry coating composition is then disposed on the tape. The slurry coating composition comprises (1) a solid pigment mixture, in the amount of from about 30% by weight to about 80% by weight of the slurry coating composition, the solid pigment mixture comprising Crxe2x80x94Al alloy containing from about 20 wt. % Al to about 60 wt. % Al of the alloy; and LiF in an amount from about 0.3 wt. % to about 15 wt. % of the Crxe2x80x94Al alloy; (2) at least one organic binder; and (3) a solvent. The tape is adapted to substantially decompose without residue upon heating to a decomposition temperature which is below a temperature effective to cause the alloy to react with the halide activator and the target surface. The target surface is heated to a temperature effective to cause the alloy to react with the activator and the target surface and thereby form an aluminide coating on the target surface.
Optionally, a masking material may be disposed onto an area of the metal substrate before positioning the coating tape. The area is laterally adjacent to the contained space and not within the contained space. The masking material inhibits the coating material from forming an aluminide coating on the laterally-adjacent area.
According to another embodiment of the invention, an article comprises a metal substrate having a target surface bounding a contained space formed by the substrate and a coating tape disposed over the contained space to at least partially enclose the space. The coating tape is in out-of-contact relation with the target surface. The coating tape comprises: (1) a mixture of (i) at least one aluminum source comprising from about 70% to about 99% by weight of the mixture, the aluminum source containing from about 20% wt. to about 60% wt. aluminum; and (ii) at least one halide activator comprising from about 1% to about 15% by weight of the mixture; and (2) at least one binder. An aluminide coating is formed on the target surface of the contained space upon heating the metal substrate to a temperature effective to cause the aluminum source to react with the halide activator and the target surface.
As used herein, xe2x80x9caluminum sourcexe2x80x9d means elemental aluminum or a compound or alloy of aluminum.
As used herein, xe2x80x9ctarget surfacexe2x80x9d means a portion of the surface of a metal substrate to be aluminide diffusion coated.
As used herein, xe2x80x9ccontained spacexe2x80x9d means is a space bounded by the target surface.